Among various factors influencing quality of a semiconductor or a FPD in a semiconductor process and a FPD process, it is important to control the thickness of a thin layer. Therefore, it is necessary to directly monitor the thickness of the thin layer during the processes. ‘A thin layer’ is a base layer, i.e. a layer formed on a surface of a substrate while having an extremely minute thickness, the thickness of the thin layer being within several tens of Å˜several μm. In order to apply such a thin layer to a specific use, it is necessary to know the thickness, composition, and physical and optical characteristics of the thin layer. Particularly, it has recently become a general tendency to form multiple-super thin layers on a substrate so as to increase the degree of integration of a semiconductor device. In order to develop such a high integration semiconductor device, it is necessary to exactly control a property of a thin layer, including a thickness thereof, which is a factor exerting remarkable influence on its property. There are various methods for measuring the thickness of a thin layer used in a semiconductor process, an application process, etc. Among these methods, a method using an interferometer and a method using a spectroscopic detector are the most typical methods.
However, in a case where objects to be measured are made from non-transparent material, the method using an interferometer can measure the almost objects, and in a case where objects to be measured are made from transparent material, it is limited for the method using an interferometer to measure the objects. Particularly, in a case where a transparent thin layer stacked on a base layer has a sufficient thickness, it is possible to distinguish an interference signal generated by a ray reflected from a boundary surface between an air layer and a thin layer from an interference signal generated by a ray reflected from a boundary surface between the thin layer and a base layer so that the thickness can be measured. However, in a case where a transparent thin layer stacked on the base layer does not have a sufficient thickness, for example, in a case where it has a thickness below micrometer, both interference signals are overlapped so that it is impossible to measure the thickness of the thin layer.
Also, according to the method using a spectrophotometer, in a measurement object to be measured, thickness in only a specific point can be measured, and it is impossible to obtain a two-dimentional shape or a three-dimentional shape of the entire surface of the object.